1. Field of the Invention
The present invention relates to a hydrogen sensor having a semiconductor layer which is sensitive to hydrogen as a function of temperature and a covering layer which is selectively permeable to hydrogen.
2. Description of the Prior Art
A general problem of gas sensors is that changes in the measured value are observed even when the gas component to be measured does not change. This is particularly disruptive if one component with a low mixture ratio or low concentration is to be measured in the presence of large but variable concentrations of gases which cause cross-sensitivities.
It has been attempted to develop a hydrogen sensor which uses gallium oxide, Ga2O3, as the semiconductive metal oxide. This substance also responds to other gases, in particular oxygen and hydrocarbons. For this reason, the sensor has at most a limited suitability for measurements in environments in which the above gas components fluctuate considerably. It has therefore been attempted to arrange a covering layer on top of the gallium oxide, which selectively allows hydrogen to pass through while other components are prevented from gaining access to the gas-sensitive semiconductor layer.
However, even with a covering layer of this type a gallium oxide sensor frequently gives only unsatisfactory results when measuring exhaust gas. This is because, in addition to the presence of interfering gas components of fluctuating composition, other factors also have adverse effects during gas measurement, in particular the measurement of exhaust gas in automobiles. These factors include the fact that electromagnetic interference etc., which may be particularly intensive in the automotive sector, frequently result in very poor measurement of the electrical parameters, such as the electrical conductivity, of the semiconductor layers used. In addition, the variation in other variables, for example the temperature of the sensor used, as a result of fluctuations in the ambient temperature or a drift in the supply of heating energy, may also have adverse effects.
German Patent Document No. 42 03 522 C1 has disclosed an oxygen sensor arrangement based on semiconductive metal oxides in which the temperature sensitivity has been reduced. When the metal oxides are at elevated temperature, the conductivity depends on the oxygen partial pressure, the sensor arrangement having two individual metal oxide sensors which, in the intended measurement area, have conductivities which exhibit different relationships with the oxygen partial pressure but substantially the same relationship with the temperature. The theory is that the temperature dependency is to be substantially taken out in the quotient formed from the conductivity measurement signals from the two sensors.
The present invention is based on the object of providing a novel arrangement for industrial use, in particular, although not exclusively, of providing a gas sensor which is particularly suitable for measuring hydrogen and can also be used in particular for exhaust gas measurements.
The invention is based on the recognition that strontium titanate, once it has been covered with a covering layer which selectively filters hydrogen, is eminently suitable for use as a hydrogen sensor and that, by way of a suitable different doping of the two layers, it is also possible to compensate for the temperature sensitivity during the measurement of hydrogen. It is assumed that the hydrogen filtering action is brought about by a selective permeability to this substance, although in particular a quasi-selective reaction of hydrogen with oxidizing substances in the covering layer also cannot be ruled out altogether. The precise mechanism of the conductivity change is actually unclear. One possible assumption is that, unlike in the case of oxygen detection using strontium titanate semiconductors, where oxygen is introduced into the crystal lattice in atomic form, where it changes the electronic properties of the lattice, in the present case a reversible chemical reaction takes place between the hydrogen and the materials in the semiconductor, and this reaction can only be detected indirectly as a change in an electrical characteristic. A further supposition is that hydrogen accumulates on the surface of the strontium titanate and as a result changes the electronic band structure of the semiconductor in the vicinity of the surface.
A preferred covering layer will contain silicon as a constituent, which can be attributed to the fact that many silicon compounds have a crystal lattice which is so tight that apart from hydrogen it is impossible for any gases, or at least any relevant gases, to pass through, and another advantage is that many silicon compounds have at most a poor conductivity within the intended temperature range of the sensors, so that the measurements of the electrical parameters of the strontium titanate semiconductor layers are at most slightly, and in practice not significantly, affected. Preferably, the covering layers consist of silicon dioxide and/or silicon nitride. Applying the covering layer as a thin film by sputtering or CVD ensures that the covering layer has a structure which is sufficiently impermeable to interfering gases.
Preferably, one of the strontium titanate layers will be n-doped and the second will be p-doped. The changes in the properties of the strontium titanate caused by hydrogen are reversible and, due to the different dominating electrical conduction mechanisms in the n-doped and p-doped layers, may typically act as opposite changes in the conductivity of the individual layers. Consequently, a reduction in conductivity is observed in one layer and an increase in conductivity is observed in the other layer.
It is preferable for the electrical conductivity of the two semiconductor layers to be measured as a function of the hydrogen concentration and then for the difference or the quotient of the corresponding measured values to be determined as the hydrogen measurement signal.